The primary objective of this F31 application is to determine the accuracy and precision of consumer activity trackers in estimating energy expenditure, activity minutes, sedentary time and steps in the free-living setting. Few consumer activity tracker validation studies have been conducted, and of the studies performed, the setting has been under controlled laboratory conditions. The new approach will use free-living conditions as the experimental setting. This project will use direct observation as the criterion measure for energy expenditure, activity minutes and steps. The accuracy and precision of the consumer activity trackers worn on the wrist and hip will be evaluated using direct observation as the source of criterion measures. We will also estimate energy expenditure during these free-living sessions using the Sensewear (SW) device as a secondary comparison measure. Having participants perform their natural activities in free-living settings will allow us to understand hw well consumer activity trackers detect energy expenditure, activity minutes and steps. We will conduct 96, two-hour free-living measurement sessions. These will be composed of three, two-hour sessions in 32, 18-59 year old adults using a previously validated direct observation system as the criterion method to measure energy expenditure, activity minutes, and steps. The SW will provide an estimate of EE and will be compared to the EE estimates from the consumer monitors. The results of the proposed project will provide evidence about the accuracy and precision of a variety of consumer activity trackers. We will also perform a comparative analysis among consumer activity trackers in estimating energy expenditure, activity minutes and steps. The proposed research plan will integrate his training strategies by applying the knowledge gained in Responsible Conduct of Research, data collection and analysis techniques, and experimental methodology which address the current knowledge gap and provide a pathway to success for the applicant.